Sight glasses are used to provide a visual indication of a fluid level. A sight glass uses the reflection of light caused by a glass to air interface, and the absence of reflection caused by a glass to fluid interface, to provide the visual indication of fluid level. Some fluids, such as gearbox oil, can form a heavy film on sight glass interface surfaces thereby giving a false fluid level reading. The absence of reflected light in such a case may cause one to assume erroneously that fluid levels are adequate. Equipment failure may result.
In helicopter pre-flight inspection, it is required that the fluid levels of various rotor gearboxes be at normal levels. The levels are checked by using a flashlight to examine the appearance of a commercial sight glass that is mounted on the side of each gearbox. Such a procedure may be difficult to perform due to the location of the sight glasses, may be complicated by adverse weather or sea conditions, and may result in false readings. New methods and apparatus of checking for fluid levels are sought.